1. Field of the Invention
The present invention relates generally to nuclear reactor vessels and, more particularly, to a device for securing a reactor vessel washer to a reactor vessel stud, allowing the stud and washer to be maintained as a mated pair during removal and replacement of the stud.
2. Description of the Prior Art
In a commercial nuclear reactor vessel, heat, from which steam and ultimately electricity are generated, is produced by fission of a fissile material, such as uranium disposed inside the reactor vessel. The reactor vessel includes a body having a cylindrical wall with two ends. The wall includes an outwardly extending flange at one end, and a semicircular domed bottom is integrally attached to the opposite end of the wall. A reactor dome is attached to the body and is positioned atop the body. The dome has a semicircular shape with an annular shaped lip portion as is well known in the art. An outwardly extending flange is disposed around the circumference of the dome at the lip portion. The body flange is positioned abutting the dome flange. A plurality of holes through the dome flange are in registry with a plurality of holes inserted partially through the body flange. Each hole in the dome mates with a corresponding hole in the body flange forming a matched pair of holes. A plurality of washers, in registry with a flange hole, are positioned over the matched holes. A plurality of studs each having two ends and a threaded shaft are used to attach the dome to the body. The plurality of studs, each with a nut attached at one end, are disposed respectively through the matched holes with the stud and washer forming a mated pair. The studs are threaded into both flanges such that one end passes through both flanges, and the other end, which accepts the nut, extends upwardly from the abutting flanges. The stud is disposed in a matched pair of holes such that the nut abuts the top of the washer. The nut and washer together function to rigidly attach the dome to the body.
Due to maintenance procedures, regulations, and the need to replenish expended fuel, the interior of the reactor vessel should be periodically inspected. To inspect the interior, the dome may be repeatedly removed. However, once the dome is initially installed, during reinstallation of the dome the stud and washer should be maintained as a mated pair. This is because the stud is stressed when it is installed, and the nut and washer, thereafter, conform to the stressed stud. Therefore, each stud, nut and washer is slightly altered and installing them on a different stud could cause them to fit improperly.
One means of removing the reactor vessel head to facilitate inspection of the interior of the vessel is disclosed in U.S. Pat. No. 4,873,760. This patent discloses an apparatus which includes a gripper and is positioned over the dome. The gripper unthreads a nut associated with a particular stud, and then an attracting device magnetically attracts the washer such that the washer is forced into the gripper and abuts the nut. The gripper places the nut and washer on a pin and repeats the process for each of the nuts to be removed. The studs are then unthreaded, and the dome is removed.
Another known method includes an apparatus, generally referred to in the art as an Automated Stud Handling and Transportation System, which is positioned on the vessel head flange. A stud remover of this apparatus completely removes the stud with the nut secured on one end of the stud. The washer is manually removed from the stud and affixed to a hook on the apparatus. Next, a movable arm for holding the stud attaches to the stud. The stud remover then moves so that it is positioned over an adjacent stud. The above process is repeated until all studs are removed. Once all the studs are removed, the dome is removed by a crane. To install the studs, the process is reversed.
A major drawback to each of the above methods is that the stud and washer are removed separately. Thus, repeatedly removing and installing the studs makes the identification of which washer should be associated with a particular stud extremely difficult. As stated above, the same nut and washer should be maintained as a mated pair each time the stud is removed and then replaced. Prudent practice dictates that if a particular stud and its associated washer and nut can't be identified, a new nut and washer should be installed. This obviously results in increased costs each time the vessel head is removed and thereafter re-installed.
Another drawback to each of the above methods is that when installing the studs a worker must carefully position each washer concentrically over the matched holes. This is a time consuming operation, and, therefore, a worker will have increased exposure to radiation in the reactor. Still further, if the washer is improperly aligned with the matched holes, the washer may be damaged as the stud is inserted.
Consequently, a need exists for a simple device for removably securing a reactor vessel washer to a vessel stud allowing the washer and stud to be maintained as a mated pair during removal and replacement of the stud.